In the joining of relatively brittle materials such as ceramics, brazing with ductile braze alloys is useful in accommodating residual stress from differential expansion. It has been previously demonstrated that niobium to yttria based material joints can be made successfully with a shrinkage fit and diffusion bonding as described in U.S. Pat. No. 4,545,799 to Rhodes et al. However, diffusion bonding has a number of disadvantages in that the joint configuration may make it difficult to apply the necessary pressure (shrinkage) and the tolerances needed for a reliable joint are very tight.
Frit seals based on calcium aluminate were also applied to the joining of a lanthana-strengthened yttria (LSY) article to another LSY article or to a niobium article (1 w/o Zr-doped) for high pressure sodium lamp applications However, this technique has been limited to relatively small parts (approximately 10 mm diameter by O.8 mm thick tube). Although these LSY to Nb frit seals have remained vacuum tight after several thousand thermal cycles, it is believed that large joints associated with windows and domes may require a braze capable of undergoing plastic deformation.